Through mask electroplating is a method for forming metal bumps and pillars in a number of processing schemes in semiconductor device fabrication. One of the standard processes that utilizes through mask electroplating involves the following steps. First, a substrate (e.g., a semiconductor substrate having a planar exposed surface) is coated with a thin conductive seed layer material (e.g., Cu, or Ni seed layer) that can be deposited by any suitable method, such as physical vapor deposition (PVD). Next, a non-conductive mask layer, such as photoresist is deposited over the seed layer and is then patterned to define recessed features, where patterning exposes the seed layer at the bottom of each recessed feature. After patterning, the exposed surface of the substrate includes portions of non-conductive mask in the field region, and conductive seed layer at the bottom portions of the recessed features.
Next, through mask electroplating (or, in the case of photoresist, through resist electroplating) follows. In through resist electroplating, the substrate is positioned in an electroplating apparatus such that electrical contact is made to the seed layer, most typically at the periphery of the substrate. The apparatus houses an anode and an electrolyte that contains ions of one or more metals, that are to be plated. The substrate is cathodically biased and is immersed into the electrolyte, where metal ions from the electrolyte are reduced at the surface of the substrate, as shown in equation (1), where M is metal (e.g., copper), and n is the number of electrons transferred during the reduction.Mn++ne→M0  (1)
Because the conductive seed layer is exposed only at the bottom portions of the recessed features, electrochemical deposition occurs only within the recessed features, and not on the field (before the recessed features are filled with metal), resulting in a number of metal-filled recesses embedded into the photoresist layer.
After electroplating, the mask is removed, e.g., by a conventional wet or dry stripping method, thereby providing a substrate having a number of free standing metal bumps or pillars.
The background description provided herein is for the purposes of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.